The present invention relates to a thin developer layer forming apparatus wherein a thin layer of a dry developer is formed on a developer carrying member, which layer is used to developer a latent image.
Conventionally, various types of apparatus have been proposed and put into practice as to a dry type one-component developer apparatus. However, in any of those types, it has been very difficult to form a thin layer of one-component dry developer, so that a relatively thick layer of the developer is used. On the other hand, the recent demand for the improved sharpness, resolution or the other qualities has necessiated the achievement of the system for forming a thin layer of one-component dry developer.
A method of forming a thin layer of one-component dry developer has been proposed in U.S. Pat. Nos. 4,386,577 and 4,387,664 and this has been put into practice. However, this is the formation of a thin layer of a magnetic developer, not of a nonmagnetic developer. The particles of a magnetic developer must each contain a magnetic material to gain a magnetic nature. This is disadvantageous since it results in poor image fixing when the developed image is fixed on a transfer material, also in poor reproducibility of color (because of the magnetic material, which is usually black, contained in the developer particle).
Therefore, there has been proposed a method wherein the developer is applied by cylindrical soft brush made of, for example, beaver fur, or a method wherein the developer is applied by a doctor blade to a developer roller having a textile surface, such as a velvet, as to a formation of non-magnetic developer thin layer. In the case where the textile brush is used with a resilient material blade, it would be possible to regulate the amount of the developer applied, but the applied toner layer is not uniform in thickness. Moreover, the blade only rubs the brush so that the developer particles are not electrically charged, resulting in foggy images.
A new thin developer layer forming method which is completely different from conventional ones has been proposed in, for example, U.S. Ser. Nos. 466,574 and 601,715, which have been assigned to the assignee of the present application. In this method, a magnetic particle confining member is provided to oppose a developer carrying member, and a magnetic brush of magnetic particles are formed by a magnetic force provided by magnetic field generating means, at a position upstream of the magnetic particle confining member with respect to the movement of the developer carrying member, whereby a thin layer of non-magnetic developer particles is formed by the magnetic brush which is constrained by the magnetic particle confining member. However, it is difficult in this method to completely constrain the magnetic particles within the developer container by the magnetic particle confining member. The magnetic particles, although the amount is very small, can leak and reach the developing station. Those leaked magnetic particles can damage the latent image bearing member or cause electrical leakage between the latent image bearing member and the developer carrying member. This results in a non-uniform developed image or unclear image.